1. Field of the Invention
The invention described and claimed herein is generally related to the production of single crystal fibers known as whiskers. More particularly, the present invention is related to apparatus and methods for continuously making whiskers by the vapor-liquid-solid (VLS) method.
2. Description Of Related Art Including Information Disclosed Under 37 C.F.R. 1.97-1.99
Crystalline fibers of materials such as silicon carbide, silicon nitride, boron carbide and boron nitride are commonly referred to as whiskers. In bulk form these materials are known for their hardness. In whisker form, they are further noted for their high tensile strength and high modulus relative to other fibrous materials, as well as their high chemical stability and high resistance to corrosion and oxidation. All of these characteristics are retained even at relatively high temperatures, making whiskers of these materials desirable as reinforcing elements in ceramics, metals, polymeric composite materials, and in other applications. Further, yarns and other woven materials made from these whiskers would have widespread utility, but there has not been heretofore available a process for making significant quantities of high quality whiskers that are also sufficiently long to be spun into yarn.
It has been previously known to prepare silicon carbide and silicon nitride whiskers by processes such as those described in U.S. Pat. Nos. 2,122,982 and 3,855,395. These processes suffer from the disadvantage that they produce a significant amount of non-fiber particulate material as well as particular impurities, which must be separated from the whisker product before it can be used.
Also, the above-referenced previously known processes produce whiskers of varying thicknesses as well as varying lengths. In this regard, it is known that whiskers having thicknesses of less than about a micron constitute a health hazard similar to that of asbestos, and a three micron thickness has been considered a minimum diameter for safe handling in the absence of special precautions. The whisker product of these prior art processes contains a significant amount of sub-micron sized whiskers, and for this reason must be considered hazardous.
Whiskers are also known to be produced by the Tokai process, disclosed in U.S. Pat. No. 4,500,504 to Yamamoto, which involves exchanging a water soluble catalyst onto a silica gel, mixing the dried silica gel with a fine particle carbon black, and reacting the mixture in a nonoxidative atmosphere at elevated temperature. This process produces a sub-micron diameter whisker.
U.S. Pat. No. 4,504,453 to Tanaka, et al., discloses the use of thin, porous silicon-and carbon-containing material, which is passed through a series of increasing temperature zones to produce sub-micron silicon carbide whiskers in the material.
The vapor-liquid-solid (VLS) process for growing whiskers is described, for example, in U.S. Pat. No. 3,622,272 to Shyne, et al., U.S. Pat. No. 3,692,478 to Knippenberg, et al., U.S. Pat. No. 3,721,732 to Knippenberg, et al., and U.S. Pat. No. 4,013,503 to Knippenberg, et al. In the VLS process gaseous compounds of whisker constituents such as silicon and carbon are introduced into a reaction chamber at an elevated temperature. There is also introduced into the reaction chamber a solid substrate that is coated with a particulate catalyst, which melts at the temperature of the chamber to form small molten beads. The gaseous reactants dissolve in the molten catalyst beads. Suitable catalysts for carbide type whiskers for this purpose include iron, manganese, nickel, cobalt, ferro silicon, stainless steel and various glasses for nitride type whiskers.
Solid silicon carbide crystallizes out of the molten catalyst at the interface between the molten catalyst and the underlying solid substrate. The silicon carbide typically crystallizes in the form of a slender whisker of solid crystalline beta silicon carbide, with the molten catalyst bead resting on and being raised upwardly on the end of the whisker as the whisker grows. The whisker is typically of a diameter similar to that of the molten catalyst bead.
Other carbide and nitride whiskers, for example B.sub.4 C, TiC, S.sub.3 N.sub.4 and BN can also be grown by the VLS process, using various transition metals, iron alloys and glasses as catalysts.
U.S. Pat. No. 4,789,537 to Shalek, et al., discloses an improvement to the VLS process, whereby a catalyst is prealloyed with silicon and carbon to accelerate the initiation of the VLS process for growing silicon carbide whiskers. A disadvantage however of using prealloyed catalysts is that prealloying lowers the melt viscosity of the catalyst, thereby causing excessive wetting and spreading of the molten catalyst, which breaks into small parts. This in turn produces a large number of submicron diameter whiskers, which as noted above are recognized to be a health hazard.
It has been previously known to grow submicron diameter silicon carbide whiskers on carbon yarn that is pulled through a tubular furnace, as disclosed in U.S. Pat. No. 3,580,731. Although the process disclosed therein allows for continuous whisker production in small quantities, it does not utilize the advantages of the VLS process, and there is also no means for controlling the stoichiometry of the reactant gases in the furnace, so that there is no means for controlling the crystal sizes or morphologies obtained by the process.